1. Field of the Invention
The present invention relates to thin film transistors with increased gate dielectric breakdown voltage, and to a manufacturing method of the thin film transistors.
2. Description of the Background Art
Insulated gate field effect transistors configured by forming a semiconductor thin film on an insulating substrate and providing a channel region in this film includes a so-called thin film transistor.
FIG. 7 is a diagram showing a sectional structure of a conventional thin film transistor. This thin film transistor is disclosed in International Electron Device Meeting 1988, for example. Referring to FIG. 7, in a conventional thin film transistor, a gate electrode 1 formed of polysilicon is formed on a surface of an insulating substrate or insulating layer 10. Furthermore, a dielectric layer 3 formed of an oxide film with a film thickness t of about 250 .ANG., for example, is formed on the surfaces of insulating substrate 10 and gate electrode 1. Furthermore, a polysilicon layer with a film thickness of about 200 .ANG. is formed on the surface of dielectric layer 3. Conductive regions 4 and 5 to be a pair of source/drain regions are formed in polysilicon layer 8, wherein a region located between the conductive regions 4 and 5 configures a channel region 2.
Next, the process for manufacturing such a thin film transistor as described above will be described. FIGS. 8A through 8D are manufacturing process sectional diagrams of the thin film transistor shown in FIG. 7.
First, as shown in FIG. 8A, after forming a polysilicon layer on the surface of insulating substrate 10, it is patterned to be a predetermined shape employing the photolithography method and the etching method. Gate electrode 1 is formed in this process.
Next, as shown in FIG. 8B, applying the low pressure CVD (Chemical Vapor Deposition) method all over the surface, an oxide film is deposited to a film thickness of about 250 .ANG., for example. The oxide film serves as dielectric layer 3.
Furthermore, as shown in FIG. 8C, a polysilicon layer 8 is formed on the surface of dielectric layer 3 to a film thickness of about 200 .ANG. using the low pressure CVD method. Then, using the photography method and the etching method, polysilicon layer 8 is patterned into a predetermined shape.
Subsequently, as shown in FIG. 8D, a resist pattern 6 is formed at a predetermined position on the surface of polysilicon layer 8. Then, using the resist pattern 6 as a mask, p-type impurity, e.g., BF.sub.2 ions 7 are implanted into polysilicon layer 8 to form p-type conductive regions 4 and 5 in polysilicon layer 8.
By the above-described process, the thin film transistor shown in FIG. 7 is formed.
A thin film transistor manufactured by the above manufacturing method has a problem that the film thickness of dielectric film 3 is uneven especially above a corner of gate electrode 1, so that a strong electric field is produced in this region in a predetermined operation. That is, in dielectric film 3 formed by the CVD method, the film thickness is decreased at the corner of gate electrode 1 as compared to other portions. Accordingly, in the vicinity of this region, an effective film thickness necessary for securing a predetermined dielectric breakdown voltage decreases.
FIG. 9 is an electric field distribution diagram showing electric field distribution in the vicinity of a corner of gate electrode. In the condition shown in the figure, -5 V is applied to gate electrode 1, 0 V is applied to a source region, and -5 V is applied to the drain region. In this simulation, a dielectric film 3 is modeled in a shape having a uniform film thickness and having a corner corresponding to a corner of gate electrode 1. As shown in FIG. 9, it is known that the gradient of the equal electric field curves 11 become sharp in a corner of dielectric layer 3 located above a corner of gate electrode 1 to produce electric field concentration. Such a strong electric field region causes leakage from gate electrode 1 to source/drain regions 4 and 5 to decrease the driving current of the thin film transistor. Also, in some cases, the strong electric field concentration produces dielectric breakdown of the dielectric film.